I recently bought a Brunton charger, just their $70 cheapy one with a single panel. The nice thing is it has a battery inside to store the power it gets, but so far I've yet to have it say it's charging at all from the sun.
Only gotten juice into it from the wall, so far. But it is winter, and the sun is low on the horizon most of the time, and there's been no real "sun" to speak of, just clouds.
I have a feeling you might need a fairly big panel to get a real charge going in any short timeframe. I bought mine for camping/backpacking, figuring I could strap it to the top of my pack for a daily charge. But I bet it won't get anything in shaded woods. Ah well...